Microglia are the first responders to central nervous system (CNS) injury or disease (Ransohoff and Perry, Annu Rev Immunol 27, 119-145, 2009). As innate immune sensors, these cells are equipped with a suite of receptors that allow them to detect disturbances in their microenvironment through the presence or absence of soluble and membrane-bound signals whose expression may be triggered by intrinsic or extrinsic events (Kettenmann et al., Physiol Rev 91, 461-553, 2011; Tremblay et al., Neurosci 31, 16064-16069, 2011). Microglia activation leads to an inflammatory response typically aimed at restricting tissue injury or pathogen spread. Under certain conditions, maladaptive innate immune responses can lead to undesired cell loss through microglial phagocytosis (Brown and Neher, Nat Rev Neurosci 15, 209-216, 2014).
A key, so-called ‘eat-me’ signal for phagocytosis is phosphatidylserine (PtdSer) (Arandjelovic and Ravichandran, Nat Immunol 16, 907-917, 2015; Lemke, Cold Spring Harb Perspect Biol 5, a009076, 2013; Sierra et al., Front Cell Neurosci 7, 6, 2013). PtdSer is a phospholipid constituent of all eukaryotic cell membranes, but displays a remarkable asymmetry in its distribution, in that it is normally confined to the inner (cytoplasm-facing) leaflet of the plasma membrane. During apoptosis or cell stress, PtdSer is externalized, potentially serving as a tag for engulfment of these cells by microglia and other professional phagocytes (Arandjelovic and Ravichandran, Nat Immunol 16, 907-917, 2015; Brown and Neher, Nat Rev Neurosci 15, 209-216, 2014; Sierra et al., Front Cell Neurosci 7, 6, 2013). Microglia can detect tagged cells, for example, through contact-mediated sensing, enabled by continual extension and retraction of microglial cell processes (Davalos et al., Nat Neurosci 8, 752-758, 2005; Fourgeaud et al., Nature 532, 240-244, 2016; Nimmerjahn et al., Science 308, 1314-1318, 2005). PtdSer orientation in the plasma membrane is regulated by calcium-dependent and -independent phospholipid transporters (Frey and Gaipl, Semin Immunopathol 33, 497-516, 2011). High intracellular calcium concentrations may promote PtdSer externalization (Segawa and Nagata, Trends Cell Biol 25, 639-650, 2015). However, the circumstances that lead to intracellular calcium dysregulation, PtdSer externalization, and microglia activation in vivo are for the most part unknown.
Viral infections of the CNS trigger innate immune responses that can include microglia phagocytosis (Sierra et al., Front Cell Neurosci 7, 6, 2013; Swanson and McGavern, Curr Opin Virol 11, 44-54, 2015; Vasek et al., Nature 534, 538-543, 2016). In particular, adenovirus (Ad)-based expression vectors are reagents for gene transfer in the CNS. Ad5-based vectors are widely used vectors in basic research applications and gene therapy. They are particularly attractive agents in the CNS where cellular division is limited, which in principle, would enable long-term transgene expression. However, Ad5-based vectors can induce potent inflammatory responses and undesired cell loss (Castro et al., Expert Opin Biol Ther 14, 1241-1257, 2014; Hendrickx et al., Hum Gene Ther 25, 265-284, 2014; Tobias et al., J Neurol Neurosurg Psychiatry 84, 213-222, 2013; Wold and Toth, Curr Gene Ther 13, 421-433, 2013). The underlying effector mechanisms remain poorly understood.